Arcing is a luminous discharge of electricity across an insulating medium, usually accompanied by the partial volatilization of the electrodes. Arcing conditions commonly occur in many electric circuits; for example, several loads and components used in households are characterized by arcing conditions in their normal operation or they absorb currents whose waveforms are similar to the arcing current, but these arcs are not considered dangerous (these are referred to as “safe arcs” in the following). Some examples of these types of loads are vacuum cleaners, lamps controlled by snap switches or dimmers, electronic variable-speed electric hand-held shop tools, and electronic switch mode power supplies. On the other hand, harmful arcing conditions (referred to as “arc faults” in the following) may occur, due to aged or damaged wires, worn electrical insulation, wires or cords in contact with vibrating metal, overheated or stressed electrical cords and wires, or misapplied or damaged electrical appliances. In an electric circuit, the effects of the arc faults may pose a risk of fire ignition under certain conditions if the arcing persists. Arc faults can also occur in DC electric circuits, for example in electric vehicles, ships, aircrafts, photovoltaic plants, variable speed drives, and so on. If not promptly detected and extinguished, such arc faults could spread to adjacent circuits, endanger power sources and control systems, cause explosions and fire ignitions.
An arc fault is usually not detected by common circuit protection devices and breakers (thermal/magnetic or differential). Thus, arc fault circuit interrupters (AFCIs) have been introduced to protect against an arc fault. The AFCI operates to mitigate the effects of arcing faults by functioning to de-energize the circuit when the arc fault is detected.
Arc faults can be divided into two categories: parallel arc faults and series arc faults.
A parallel arc fault occurs between the line and neutral conductors (line-to-neutral fault) or even between a conductor and ground (line-to-ground or neutral-to-ground). In these cases the load is shortcut and the arcing current usually assumes values higher than the normal values for the electric circuit, being limited only by the source power and the circuit impedance. The detection of such a parallel arc fault is a rather straightforward issue because of the increase in current amplitude. In many cases, the current amplitude rises above the tripping threshold of the magnetic/thermal protection so that the parallel arc fault can be extinguished using the common circuit protection devices.
On the other hand, a series arc fault occurs in a single conductor due to damage present on that conductor. In most cases, the arc fault occurs in series with a load whose impedance reduces the amplitude of the arcing current to within the normal values for the electric circuit. Thus, current in the circuit will not rise to a level which trips common circuit protection devices. The recognition of a series arc fault and its discrimination against normal conditions can be very difficult, for example, this is the case with electric circuits, where loads producing “safe arcs” (i.e., “masking loads”) are present in the circuit involved by the arc fault.
Reference is now made to FIG. 1A which illustrates a typical waveform of an arcing current and FIG. 1B which illustrates a typical waveform of a non-arcing current (both obtained for an AC electric circuit, in the presence of a resistive load). It will be noted from FIG. 1B that normal operation exhibits a current waveform having a sinusoidal shape. In the presence of a series arc fault condition, however, FIG. 1A shows that the sinusoidal shape has been disturbed. More particularly, the current waveform is characterized by a rate of rise in the arc current (see, reference 101) that is usually greater than that present with normal current (see, reference 102). Furthermore, in each half cycle, regions of no current flow, referred to in the art as “shoulders” (see, reference 103), are present in the current waveform ahead of each steep rise in current. These zero-current regions occur due to the fact that the arcing current extinguishes before a normal current zero crossing (104) and reignites with a certain delay following the zero crossing. Other arcing characteristics not explicitly shown in FIG. 1A, but known to those of skill in the art, include the following: the amplitude of current is generally lower in the presence than in the absence of an arc (because of the voltage drop across the arc); the current signal exhibits broadband high-frequency noise in the presence of an arc; and the arcing phenomenon is non-stationary and frequently sporadic (thus segments of arcing signal can be mixed together with segments of normal non-arcing current and also with zero-current periods).
One or more of the aforesaid arcing characteristics, which may be clearly visible in the case of a resistive load, can be hidden in the presence of loads producing “safe arcs”. By this it is understood by those skilled in the art that current waveforms may look very similar to each other in the presence and in the absence of an arc fault, especially where the load is a masking load. Reference is now made to FIG. 2A which illustrates a typical waveform of an arcing current and FIG. 2B which illustrates a typical waveform for a non-arcing current (both obtained for an AC electric circuit, in the presence of a load including a dimmer with tungsten lamps). It will be noted that shape of the current waveform, for example in terms of amplitude, shoulder shape and shoulder presence (see, references 203 and 204) and rise time (see, references 201 and 202) are quite similar. The presence of the arcing condition in FIG. 2A can be quite difficult to detect in comparison normal operation in the presence of a masking load. Conversely, there is a risk of mistakenly detecting (a false positive detection) the presence of an arcing condition in FIG. 2B.
The prior art is replete with solutions for making arc fault detections. Generally speaking, the prior art solutions function to analyze the current signal (and in few cases also the voltage signal) with the aim to identify the above mentioned characteristics of an arc fault and to distinguish the presence of these conditions from normal conditions (even with respect to “safe arcs”). The prior art solutions for arc fault detection are essentially based on: the analysis of the shape of the current waveform and its first derivative (di/dt) to individuate typical arcing characteristics such as peaks, shoulders and high rates of rise; the analysis of the high frequency broadband noise present in the current waveform; and the analysis of the non-periodicity of the signal by means of cycle to cycle waveform comparison or comparison of the waveform to a reference waveform shape obtained from the observation of the signal in a number of prior cycles.
In order to improve detection operation, the prior art solutions typically make use of more than one of the detection methods, with the outputs combined and evaluated to make the detection decision. For example, an arc fault is not declared unless a number of arcing characteristics are simultaneously observed. This results in a reduced risk of unwanted trips or failures to trip.
The prior art solutions may further evaluate not only the first derivative of the current waveform but also on the second derivative.
It is further well known in the art to evaluate the high frequency components (from a few kHz to hundreds of kHz or even MHz) of the current waveform (and its derivatives) to recognize the presence of broadband noise and to discern the typical characteristics of an arcing current, such as the shoulders, the presence of peaks or rates of rise higher than a specified limits.
Further solutions combine the broadband noise information with the analysis of the fundamental component of the current, in order to detect the presence of the shoulders. In many cases filtering techniques are used for these purposes, since digital signal processing techniques would require high sampling frequencies in order to perform a correct measurement of the considered high frequency components.
Generally speaking, the arcing characteristics are detected by comparing the signal waveform with that of a typical arcing condition. This can be accomplished by means of a comparison of measured characteristics with predetermined thresholds. In many cases the current signal is converted into sequences of pulses generated when a predetermined arcing characteristic exceeds the predetermined threshold. The pulses are then counted up to a given threshold which is meant to identify the arcing condition. In other cases the pulses are used to charge a capacitor and in this case the threshold is a predetermined level of charge of the capacitor.
Reference is made to U.S. Pat. Nos. 5,682,101, 6,246,556 and 6,259,996 (the disclosures of which are incorporated by reference) which propose detection systems based on counting pulses (within a selected time interval) generated when a current rate-of-change signal exceeds selected thresholds in proper frequency ranges (where the frequency ranges are typical of arc faults).
Reference is also made to U.S. Pat. No. 5,839,092 (the disclosure of which is incorporated by reference) which proposes a detection system based on counting (within a selected time interval) the changes on slope of the current which are obtained by monitoring the peak currents in a series of half cycles of the waveform. In addition, current samples are normalized and self-correlated to detect waveform shape, with the number of significant waveform shape changes between consecutive half cycles being counted and evaluated to identify arc faults.
Reference is further made to U.S. Pat. No. 7,068,480 (the disclosure of which is incorporated by reference) which proposes analysis of the di/dt signal to determine the presence of broadband noise in a predetermined range of frequencies, as well as the presence of current peaks and high rates of rise. The arc fault detection is made by comparing such characteristics with predetermined values which are related to the arcing condition and to the load recognition.
U.S. Pat. No. 5,185,684 (the disclosure of which is incorporated by reference) proposes a current sensing solution where the arc detection system includes a frequency responsive circuit (with a number of band pass filters, essentially in the broadband noise frequency range) for sensing the electromagnetic field generated by the current signal. A plurality of frequencies are monitored in combination for information indicative of the presence of an arc fault.
In U.S. Pat. Nos. 5,452,223 and 5,561,605 (the disclosures of which are incorporated by reference), a harmonic notch filter samples current at a plurality of phases and cycles, and differences between two sampled currents are processed in a synchronous summer over a number of cycles. The arc fault is detected by evaluation of given conditions of the cumulative current difference signal provided by the summer.
With reference to U.S. Pat. Nos. 5,691,869 and 5,963,405 (the disclosures of which are incorporated by reference), arc fault detection is made by analyzing the waveform and amplitude of the current. A filter generates pulses whose amplitude is proportional to the amplitude of the step increase in current generated by the striking of the arc. The pulses are rectified and, when exceeding a given threshold, they are integrated by an RC circuit, which generates a trip signal when the charge voltage of the capacitor exceeds a predetermined threshold.
In U.S. Pat. No. 5,805,397 (the disclosure of which is incorporated by reference), an arc fault is detected by means of a multi-channel system using non-overlapping band pass filters generating outputs responsive to white noise produced by the arc fault. The filter outputs are logically combined by comparator circuitry with common pull-up resistors or by analog multipliers to producing the trip signal for the breaker.
Referring now to U.S. Pat. No. 5,815,352 (the disclosure of which is incorporated by reference), an arc fault detector includes a pulse generator which provides a pulse each time an arc is struck. When a time attenuated accumulation of pulses reaches a predetermined threshold, the trip signal is produced. A limiting amplifier limits the amplitude of the pulses to discriminate against false trips.
In U.S. Pat. No. 5,818,237 (the disclosure of which is incorporated by reference), a signal conditioner generates a bandwidth limited di/dt signal having pulses produced by current step increases. A first circuit tracks the envelope of the di/dt signal with a first time constant and a second circuit tracks the di/dt envelope with a shorter time constant. The arc fault is detected when the second tracking signal falls to a predetermined fraction of the first tracking signal.
U.S. Pat. No. 5,835,321 (the disclosure of which is incorporated by reference) teaches a band-pass filter used to generate an arcing current signal having a bandwidth of about 3 kHz to 20 kHz. For each cycle that the arcing signal exceeds a threshold, preferably related to the AC current amplitude and for a selected duration of the cycle, a fixed pulse is generated. If a time attenuated accumulation of these fixed pulses reaches a predetermined level, the arc indicative signal is produced.
U.S. Pat. No. 6,388,849 (the disclosure of which is incorporated by reference) teaches an arc fault detector including an average instantaneous current generator that averages the current over the fundamental period and produces an output indicative of an arc fault when substantial variations in the current waveform are present between half-cycles. To discriminate over inrush currents, a pulse generator produces a pulse in response to step increases of the current caused by an arc striking. An output circuit generates an arc fault signal when the time attenuated accumulation of pulses occurring in half-cycles, in which the average instantaneous current is above a selected threshold, reaches a predetermined level.
Other prior art solutions are based on the analysis of typical frequencies and characteristics related to the arcing current.
For example. U.S. Pat. No. 5,706,159 (the disclosure of which is incorporated by reference), teaches an arc fault detection system including two swept filters and associated amplifiers which produce a signal whose amplitude is representative of high frequencies (of about few Mhz to around 20 Mhz) present in an arcing current. The portion of the filtered signal having an amplitude above a predetermined value is integrated and the trip signal is produced when the integration exceeds a predetermined limit.
In U.S. Pat. No. 5,729,145 (the disclosure of which is incorporated by reference), arc fault detection is based on an analysis of wideband high frequency noise which exhibits distinctive patterns of variation in amplitude synchronized to the gaps (shoulders) in the power waveform.
With reference to U.S. Pat. No. 6,031,699 (the disclosure of which is incorporated by reference), arc fault detection is based on a simultaneous analysis of current peaks and variations. A level detector generates a first pulse when the current exceeds a first level and a step detector generates a second pulse when a rapid step increase of the current exceeds a second level. The first and second pulses are combined and an arc fault is generated when the combined pulses exceed a third level.
U.S. Pat. Nos. 7,110,864 and 7,307,820 (the disclosures of which are incorporated by reference) combine a number of the aforesaid solutions to examine a plurality of arcing events related to the amplitude-duration pair or to the broad-band signal current characteristics.
Other proposed solutions for arc fault detection are based on methods of frequency analysis of the di/dt signal. For example, U.S. Pat. No. 6,198,611 (the disclosure of which is incorporated by reference) teaches passing the di/dt signal through a high pass filter to attenuate the fundamental current component. The filtered signal is then integrated and a trip signal is produced if the integrated signal exceeds a threshold level. A current transformer is used to sense the di/dt signal. The current transformer saturates at a predetermined current level to discriminate signals with normal and high di/dt.
In U.S. Pat. No. 6,362,628 (the disclosure of which is incorporated by reference), a pulse width modulation (PWM) technique is used to detect the broadband noise associated with the arcing current. A logic signal is created having a duration or width corresponding to the time intervals during which broadband noise is present. The random starts and stops of the arc fault modulate the width of the logic pulse with respect to the current zero crossing. A microprocessor monitors the PWM logic pulse and increments a counter when a difference between two consecutive pulse lengths exceeds a predetermined amount. If the counter reaches a predetermined value, associated with the arc current amplitude, the trip signal is produced.
U.S. Pat. No. 7,227,729 (the disclosure of which is incorporated by reference) teaches making an arc fault detection based on an analysis of both the current and its derivative characteristics. The high frequency components of the input current are processed to detect the peak, rms and/or average values of the current. Moreover, the input current is filtered and rectified and the di/dt characteristics are obtained. A processing unit correlates the absolute current and the di/dt characteristics to distinguish between arc faults and nuisance loads.
With reference now to U.S. Pat. No. 6,088,205 (the disclosure of which is incorporated by reference), the arc fault detection is made based on an analysis of both the line frequency component and the high frequencies components of the current. The line frequency component provides an indication of the amount of the input current. The high frequency components are indicative of the level of arcing. If the average high frequency signal is grater that the level expected for a normal appliance arcing at the associated average line frequency level, then the trip signal is generated.
With further reference to U.S. Pat. No. 6,972,572 (the disclosure of which is incorporated by reference), the arc fault detection is made by analyzing the peak values of the di/dt signal using a peak detector with decay.
U.S. Pat. No. 7,003,435 (the disclosure of which is incorporated by reference) teaches an arc fault detection made based on a simultaneous evaluation of different characteristics of the current. Apart from the current waveform, two pulse signals are obtained for each occurrence of positive or negative step changes in the current with di/dt values higher than a predetermined value. Moreover, the broadband noise level in the current and the zero crossings is analyzed. A microcontroller compares these characteristics to determine whether the arc fault is present.
U.S. Pat. No. 7,062,388 (the disclosure of which is incorporated by reference) teaches a frequency harmonic identifier that detects series arcs using a Fast Fourier Transform (FFT) based technique to provides the harmonic content of the sensed current signal. Reference information relating to a variety of common loads is stored. Circuit logic functions to compare the sensed harmonic content to the reference information. In the absence of a match, a series arc fault signal is generated.
Those skilled in the art will recognize a number of limitations and drawbacks associated with the foregoing solutions of the prior art. For example, although solutions are presented to distinguish arc faults from certain known safe arcs, these solutions continue to suffer from instances of false arc fault detection. Additionally, the commonly utilized broadband signal analysis for arc fault detection requires the use of analog circuit solutions, mainly based on filtering. Digital solutions would be preferred but are not satisfactory because of a required high sampling rate and signal processing frequency and a wide observation window on a stationary signal (due to the use of spectral analysis algorithms such as traditional FFT which provide improved spectral resolution) and this conflicts with the sporadic and non-stationary nature of the arcing phenomenon over a wide observation window. On the other hand, if the observation window is made small, the spectral resolution is poor. In the cases which employ a low frequency signal analysis, that analysis is typically applied to the derivative of the current signal (di/dt) or it is used to fix some thresholds for the di/dt analysis and the discrimination of the arcing conditions against good arcs. In other cases, the low frequency analysis is used to compare the harmonic content of the current with predetermined reference signal bands, which may represent common loads. This solution requires a priori knowledge of the electric circuit, in each and every working condition, in order to determine the reference signal bands representing the possible load configurations. Furthermore, signal acquisition it typically accomplished using current transformers which may have a poor frequency response and a low signal-to-noise ratio. Better results may be obtained with shunts.
In spite of numerous and varied solutions for make an arc fault detection, there remains a need in the art for an improved arc fault detection circuit and method.